EM 1110-2-1100 (Part II)
30 Apr 02
e. Evaluating inlet hydraulics with Keulegan K.
(1) Introduction. The Keulegan K value can be used to understand effects of gross geometric variations
on inlet hydraulics. Figure II-6-24 shows ocean and bay tide curves and velocity in the channel during a tidal
cycle. Three K values are represented. The lower the K or filling coefficient, the lower the tide range in the
basin and the greater the phase lag (or time difference between high water (low water) in the ocean and high
water (low water) in the bay. Also noted from the velocity curve, maximum current occurs during maximum
head difference between ocean and bay tides and as K decreases, maximum ebb and flood currents shift from
occurrence at a mid-tide level to tidal elevation extremes. For K = 0.2, maximum flood currents occur near
high water and maximum ebb currents occur during low water. Therefore for low K inlets, flood flow occurs
when depths in the channel and over tidal deltas are greatest (near high water), and flow is more broadly
distributed. During ebb flow, depths are shallower (approaching low water) and flow is likely to be more
channelized. As K increases, and the bay fills more completely, peak ebb and flood flows tend to occur near
the same tide level. These variations can have implications with regard to sediment transport. Mota Oliveira
(1970) found that, using an analysis similar to Keulegan but with variable entrance cross-sectional area, when
0.6 < K < 0.8, the inlet had maximum sediment flushing ability. When K > 0.8, there was flood dominance
in sediment transport capability, meaning net bayward transport and when K < 0.6, there was ebb dominance
of sediment transport capability. Also when a variable bay surface area was considered, ebb sediment
transport was enhanced to the detriment of flood transport. Part II-6-2, paragraph h contains more
information on flow dominance.
(2) Jarrett's classification. Jarrett (1975) performed a "hydraulic classification" of inlets based on
Keulegan's K after determining tidal characteristics of a large number of U.S. inlets (see Table II-6-1). The
classification is as follows:
(a) Class I: Keulegan K < 0.3. Phase angle (ε) equal to or greater than 70 deg. For a semidiurnal tide,
the phase lag between the ocean tidal extreme and slack water is equal to or greater than 2 hr 25 min. For the
most part, bays associated with inlets in this class are open and relatively shallow with only one relatively
small inlet (i.e., small ratio Ac/Ab connecting the ocean with the bay. This class also could include relatively
long estuaries that are actually long-wave embayments.
(b) Class II: Keulegan K > 0.80. Phase angle (ε) equal to or less than 40 deg. For a semidiurnal tide, the
phase lag would be equal to or less than 1 hr 25 min. Bays associated with this class are either short or
consist of a system of relatively deep channels (Note: by virtue of the long tidal period in the Gulf of Mexico
most Gulf Coast inlets fall into this class irrespective of bay shape). The ratio Ac/Ab is relatively large for
these inlets.
(c) Class III: 0.3 < K < 0.8. Phase angle lies between 40 and 70 deg. Characteristics are intermediate
to classes I and II. Analysis by Mota Oliveira (1970) indicated that the natural flushing ability of a vertical
bank lagoon reaches a maximum for K values of 0.6 to 0.8.
f.
Effect of freshwater inflow.
(1) If a significant amount of fresh water is introduced into the bay relative to the tidal prism of the inlet,
there will be significant changes in bay tide range and velocity through the inlet. King (1974) included
freshwater inflow into a Keulegan type model (which did not include the inertial term in the 1-D equation
of motion). He defined a dimensionless variable
II-6-26
Hydrodynamics of Tidal Inlets
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